Polyol esters (POEs) are well known in the art as lubricants for displacement type refrigeration systems. Commonly used commercial POEs are derived from the reaction of a polyol (an alcohol containing 2 or more OH groups) with one or more monofunctional carboxylic acids. Such polyol esters are especially suited for use in systems utilizing hydrofluorocarbon refrigerants (HFCs), such as R-134a and related molecules, because their polar nature provides improved miscibility with the refrigerant in comparison to other lubricants such as mineral oils, poly-alpha-olefins, or alkylated aromatics. One example of such a polyol ester lubricant is disclosed in U.S. Pat. No. 6,221,272.
Dipentaerythritol (DiPE) is a key polyol ingredient in the manufacture of premium polyol esters for use as refrigeration lubricants. However, the supply of DiPE is highly dependent on the demand for monopentaerythritol (PE) since DiPE is a fractional by-product of PE manufacture. At certain times, the demand for PE drops and the supply of DiPE is very limited or non-existent. There is therefore a need to identify ways to reproduce the composition and performance of polyol esters derived from DiPE without having to use this expensive and possibly unavailable ingredient.
According to the present invention, a polyol ester composition has now been developed which is produced from PE as the polyol starting material but which has similar composition and properties as a polyol ester derived from DiPE. Moreover, by controlling the composition of the carboxylic acid mixture used to react with the PE, it is possible to produce ester compositions over a range of kinematic viscosity values but all having a high viscosity index.
U.S. Pat. No. 3,670,013 discloses a process for making a partially esterified poly(neopentylpolyol) product, which comprises introducing neopentyl polyol material, aliphatic monocarboxylic acid material and a catalytic quantity of acid catalyst material into a reaction zone, whereby a reaction mixture is formed, said neopentyl polyol material consisting essentially of at least one neopentyl polyol represented by the structural formula:
in which each R is independently selected from the group consisting of CH3, C2H5 and CH2OH, said aliphatic monocarboxylic acid material consisting essentially of at least one aliphatic hydrocarbon monocarboxylic acid, and said acid catalyst material consisting essentially of at least one acid esterification catalyst, wherein the initial concentration of said aliphatic monocarboxylic acid material in said reaction mixture is such as to provide an initial mole ratio of carboxyl groups to hydroxyl groups in the reaction mixture of from about 0.25:1 to about 0.5:1, and, while said reaction mixture is established and maintained at 170-200° C., aliphatic monocarboxylic acid vapor and water vapor are withdrawn from said reaction zone. The resultant partial esters are said to be useful as intermediates in the synthesis of the corresponding poly(neopentyl polyols), such as dipentaerythritol, and in the synthesis of the corresponding fully esterified poly(neopentyl polyols).
In addition, U.S. Pat. No. 5,895,778 discloses a synthetic coolant/lubricant composition comprising an ester mixture of: about 50 to 80 weight percent of polypentaerythritol ester formed by (i) reacting pentaerythritol with at least one linear monocarboxylic acid having from 7 to 12 carbon atoms in the presence of an excess of hydroxyl groups in a mole ratio of carboxyl groups to hydroxyl groups in the reaction mixture in a range from about 0.25:1 to about 0.50:1 and an acid catalyst to form partial polypentaerythritol esters and (ii) reacting the partial polypentaerythritol esters with an excess of at least one linear monocarboxylic acid having from 7 to 12 carbon atoms, and about 20 to 50 weight percent of a polyol ester formed by reacting a polyol having 5 to 8 carbon atoms and at least two hydroxyl groups with at least one linear monocarboxylic acid having from 7 to 12 carbon atoms, the linear acids including less than about five weight percent branched acids with the weight percents of the esters in the blend based on the total weight of the composition.